The present study is concerned with the compressibility effects appearing in the motion of an unsteady flow that is initially subsonic. A fundamental flow, used in experiments, is the flow following behind the shock wave emitted from the open end of a shock-tube. This flow has a vortex ring at the head of the unsteady flow discharged continuously out of a shock-tube.First, the dynamic motions of a fundamental flow were studied by means of an optical shadowgraph method. Next, the phenomena appearing in more complex flows, produced by the interactions of a fundamental flow with other variables, were studied by the same method. The following results are discussed briefly below.The core of a head vortex ring includes a inner core with strongly concentrated vorticity and a vortex sheet surrounding it. These two sections become turbulent-like independently, during the process of the transition from a lamminar to a turbulent vortex ring. Afrer rearward-facing shocklets are generated within the head vortex ring, a very thin vortex ring appears in front of the head vortex ring. The thin vortex ring reverses its position to the rear of the head vortex ring reverses its position to the rear of the head vortex ring. The unsteady flow behind the head vortex ring, is excited by a weak shock wave, which results in the production of small vortex rings. These small vortex rings subsequently interact with each other, and then these small vortex rings merge with the adjacent ones.Supersonic zones develop within more complex flows, and shocklets are generated. The point of focus, at which the pressure increases dramatically, is produced as a result of these interactions between the shocklets and the small vortex ring.These results indicate that the unsteady flow following behind a head vortex ring makes significant works for the compressibility effects appearing in the present flow fields.